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Northwest Geology Field Trips, by Dave Tucker, is licensed under a Creative Commons Attribution-Noncommercial- Share Alike 3.0 United States License. You can use what you find here, repost it with attribution to the author, "remix" it for your own purposes, but may not use it with the intent of making money off of it.

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Baker River drains the wilderness heart of the North Cascades east of Mount Shuksan. The trail ascends the narrow river bottom from the head of Baker Lake Reservoir 2.5 miles to the banks of Sulphide Creek. It is a study in greens: the bedrock is greenschist, the trees are coated in glowing green moss, and pools along the river are filled with green water. In fine weather, there are mountain views. Do it in the murk and rain of early December, as I did with John Scurlock, and you’ll perforce focus mostly on things close at hand. While the round trip is only 5 miles, there is enough diverse geology to take up at least half a day.

Baker River trail (red dashes) begins at the head of Baker Reservoir. Sheeseh, even the map is green!

Getting there: The paved Baker River Road branches north off State Highway 20 16.5 miles east of Sedro Woolley. The trailhead is at the end of the Baker Lake Road, 26 miles from Hwy 20 and three miles beyond the end of the pavement. If you are westbound on Highway 20, Baker Lake Road is 6 miles west of Concrete. Alternatively, from Concrete take the Burpee Hill Road and reach the Baker Lake Road in 4 miles (6.6 km). To do this, turn north by the old cement storage silos on Superior Avenue at the west edge of Concrete, then turn left on Burpee Hill Road at the T-intersection in a quarter mile. If coming from the west, head east up Highway 20 from Sedro Woolley. Turn north (left) on Baker Lake Road and reach the trailhead in 19.8 miles. Trailhead elevation is 750 feet (~230 m). There is an outhouse at the trailhead. The Northwest Forest Pass is required. The trail is moderate, with only a few short ups and downs.

Topographic map: Mount Shuksan 7.5 minute sheet covers most of the trail, beginning just south of the Baker River footbridge, which is just north of Blum Creek.

The trail sets off on an old road, and soon reaches the shore of the Baker River. Glacial ice came this way from the BC interior at least six times over the past million years or so; ice abandoned this valley some time around 12,000 years ago. More recent rock slide rubble lies on the glacial sediment, along with alluvial sediment dropped by the Baker River as it reestablished itself in post-glacial times. There is little bedrock along the trail, though there are many large blocks fallen from the valley walls. All the bedrock is in the 3/4 mile or so. Generally it is covered in moss, ferns, algae, and lichen, all very green. In the woods, even seemingly bare surfaces have a thin coating of lichens, so it is pretty hard to examine actual rock surfaces.

Foliation is nearly planar in this exposure on a boulder just above the big bridge.

Shuksan Greenschist

Some low, water-smoothed outcrops in the river bed afford the best place to see bedrock. About 100 feet before you reach the big footbridge at the junction with the Baker Lake Trail, ½ mile (800 m) from the trailhead, walk out to the riverside and head for the bridge footings. Before you get there, you’ll see the slabs. The rock is Shuksan Greenschist, and is metamorphosed basaltic lava. The parental Shuksan lava erupted onto the floor of the Panthalassa Ocean around 160 million years ago. The Panthalassa (Greek, ‘all ocean’; sometimes called the ‘paleo-Pacific’) was the global ocean surrounding the Pangaea supercontinent. The supercontinent began to break up at about the time the lava was erupting. The North American plate began its westward journey (continuing today) and the western margin of the continental plate swept up small bits of seafloor and volcanic islands during subduction. Many different terranes were thus accreted to the outer edge of the continent, and the Shuksan is part of one of these, the Easton Terrane. The basalt and its overlying coating of sea floor sediment were carried beneath North America and subjected to pressures of around 7-9 kilobars, equivalent to 23-30 km (1 kb ~ 3.3 km) and temperatures of 330-400C (Brown, 1986). The rock was metamorphosed by the heat and pressure, became accreted to the margin of North America, and then was rapidly uplifted to upper crustal levels. During metamorphism, minerals were stretched out and aligned, so the rock may look layered in places. This is metamorphic foliation, not sedimentary bedding. The green color is from the metamorphic minerals, principally chlorite, actinolite, and epidote, that replaced the original minerals in the lava. The rock was deformed, and in places was folded or stretched out. It was never hot enough to melt. Wavy veins of quartz help accentuate the deformation of the greenschist. The quartz probably grew in fractures during the earliest stages of metamorphism, before the rock had been carried very deeply by subduction, and was still relatively cold and brittle.

Hard to get any greener than this! These foliated slabs are just south of the bridge on the west bank.

John Scurlock photographs the eroded tree stump in growth position beside the greenschist slabs.

While you are down at the river, see if you can find the huge eroded roots of a tree emerging from the river gravel; it is right next to the best outcrops of greenschist. The stump is in a growth position, indicating that it was not drifted here but is in situ. The remains of this tree are evidence that the river channel has migrated across the valley floor.

The problem of Lake Creek’s sediment fan

John Scurlock at the Lake Creek foot gbridge. The stream has cut down into its own fan.

Stay on the Baker River trail- don’t turn right across the bridge. You’ll pass a few places where recent debris flows and floods (perhaps in 2006) left a pavement of fist-sized stones on the forest floor. Trees and heartier shrubs poke out of the rubble. In around ¾ mile, cross Lake Creek (footlog when we were there) draining Shuksan Lake, perched in a hanging glacial cirque at 3694 feet. Notice that the creek has cut deeply into its own forested debris fan, leaving high banks of alluvial sand and cobbles. Walk a bit further and find yourself in a clearing at the top of the river bank, 20 feet or so above the gravel bars of the river. Look back downstream. Find a vantage point where you can just see the mouth of Lake Creek. Notice that the river bank slopes downward away from this tributary creek, especially evident downstream from the mouth of the creek. If a creek deposits a fan, then erodes a channel into it, this indicates that the base level of the creek has changed. Base level is the elevation at which a creek ceases to erode because it has reached the level of its mouth. Ultimate base level is sea level, but here, while the creek was depositing the alluvium several tens of feet above the modern Baker River level, the creek’s outlet (intermediate base level) in the Baker River valley must have been at a higher elevation than it is now. Something happened, and the creek’s entry into Baker River changed to a lower elevation, so it ate downward into its own sediment deposit. What was the “something”? We are at about 840 feet above sea level here, which may have particular bearing on this problem.

Lake Creek enters the Baker in the middle ground. The sloping surface of the fan/delta is apparent, well above the river level.

Back at the end of the Pleistocene, the Baker River’s mouth down at the Skagit was blocked by a huge fan of glacial outwash called the Burpee Hill Fan- you surely noticed the sediment within this fan in roadcuts if you drove up the Burpee Hill Road from Concrete. This was sediment washed off the thick lobe of the Vashon ice sheet that pushed up the Skagit valley from the Salish lowland. The sediment spilled into the Baker valley, which was dammed by the ice to form Glacial Lake Baker (Scott and Tucker 2006; Tucker and Scott, 2009). The ice receded, leaving behind a remnant well into the Holocene, drowning the Baker River valley to an elevation of— 850 feet! Ta da! So Lake Creek’s alluvial fan may have extended into this lake as a delta, and the creek’s intermediate base level was 850’. After drainage of the lake, in the mid-Holocene, Lake Creek’s base level was lowered to that of the reformed river flowing across the newly exposed lake floor, and the creek cut down into its own alluvial fan/delta.

Baker River

While you are at this river bank viewpoint, look at the Baker River. You are no doubt familiar with sweeping, looping meanders in rivers, such as the flatter portions of the lower Skagit and Nooksack Rivers as water slowly flows toward the ocean. At this vantage point there are several channels, winding around, or cutting across, gravel bars. The bars consist of sediment carried along the river’s floor in times of high water and high energy. This is a good example of a braided river, which is most common where bedload (the coarser component of a stream’s sediment load) is dominant. Look for logs stranded on these bars or along the river banks- these mark the river’s surface levels during floods. The river is forced to flow around its own sediment when water levels recede. The cobbles in the river come from a number of different rock units up stream, plus a fairly high proportion of erratics from beyond the Baker River drainage, left behind when the Baker River lobe of the various Pleistocene Cordilleran ice sheets melted away.

The unnamed waterfall.

Unnamed waterfall

The trail now leaves the river for good. Watch for a high waterfall cascading over the sheer rock wall west of the river about 0.4 mile (600 m) beyond the river bank view point. It is especially obvious if you are here when the trees are leafless. You can hike about 0.1 mile (150 m) cross country up the alluvial fan to this unnamed waterfall. You may spot a faint foot path leaving the trail on the left at or near 48° 46.398N and 121° 32.698 W. But if you don’t, then just push your way up through sparse salmonberry brush and an elfin forest of moss-coated big leaf maples. It is pretty easy going. You’ll gain 200 feet and be at the base foot of the fall on an eroded cone of landslide rubble. This is a very worthwhile side trip.

Properly dressed geo-explorer for an early December visit to the Elfin Forest below the Unnamed waterfall. (That’s John Scurlock.)

The water flows over Shuksan Greenschist. The visible part of the falls is a few hundred feet high, but the topo map indicates it begins at around 3400 feet. There are a few channels eroded by the unnamed stream. Even when we were there, in the wet season of early December, the stream itself disappeared into its own alluvial fan.

Landslide rubble

This greenschist boulder lying on the alder tree shows that debris continues to come off the valley walls.

Just beyond the landslide, the trail passes through a concentration of auto and even bus-sized blocks of fern-draped greenschist. The near-vertical valley walls soar just beyond the pile of blocks. The Baker River flows in a classic U-shaped glacial valley, with a flat valley floor filled with coarse alluvium and a steep walls, scoured by the glacial rasp during repeated Pleistocene glacial advances from the interior of British Columbia.

Sulphide Creek alluvium

Rocks at Sulphide Creek: two varieties of Easton Schist at top; volcanic breccia at right; granodiorite at bottom. Greenschist is ubiquitous.

Pass the sign at the south boundary of North Cascades National Park, and walk the level trail through soggy forest to the bank of Sulphide Creek. This creek drains the entire south flank of Mount Shuksan, including the large Sulphide and Crystal Glaciers. This is the end of the trail. Poke around among the rocks at the creek. Greenschist is the most prominent rock, but you’ll also find granodiorite or tonalite eroded from the Chilliwack Batholith. There are a couple of small granodiorite intrusions up Sulphide Creek (Tabor and others, 2003), but the contact of the Shuksan Greenschist with the main body of this huge complex of intrusive rocks is another couple of miles up the Baker River valley. You’ll also see some dark, swirled schist with folded quartz veins. This is Easton Schist, the metamorphosed sea floor sediments that were subducted along with the Shuksan Greenschist. Especially watch for sparse blocks of volcanic breccia: a dark gray or black matrix containing angular fragments of lighter rocks. It is a good idea to break one of these open with your ever-present rock hammer. The dark part of these rocks is microscopic volcanic ash and crystals. The large clasts (rock fragments) are a mix of schist, maybe some Chilliwack granitics, chunks of quartz, and very fine grained older volcanic rocks. There is a small outcrop of rock like this near the top of Cloudcap Peak (map unit Thmb of Tabor and others 2003), drained by Sulphide Creek; it is probably related to the Hannegan caldera (Tucker, 2006; Tucker and others, 2004). The caldera itself is a great mass of breccia on the other side of Cloudcap Peak, just outside the Sulphide Creek drainage area. The Hannegan rocks fill a 3.72 million-year-old collapse caldera, similar to the younger Kulshan caldera between Mount Baker and Mount Shuksan (Hildreth, 1996; Hildreth and others, 2003). The breccia formed as gigantic ash eruptions tore upward through bedrock, carrying fragments of the older rocks with it. The mix of ash and rock fragments filled the subsiding caldera structure: Hannegan caldera contains 1000 m of fill, some of it similar to these volcanic breccia blocks on the bank of Sulphide Creek.